1. Technical Field
The present invention relates generally to output buffer pre-drivers, and, more particularly, it relates to pre-driver transition control circuitry that controls the transition rate of the controlling terminal of an output driver.
2. Related Art
Conventional pre-driver circuitry presents significant problems in the context of power/ground noise. To switch an output signal with respect to its voltage level, a turning on of output drive current is required, representing a change in current. As this current must flow through the combined inductance of an integrated circuit (IC) package and the power or ground trace on a circuit board on which the IC is placed, the rate of change of current (dI/dt) produces a voltage signal on the power or ground trace on the circuit board. Undesirable Electro-Magnetic Interference (EMI) noise is emitted from power/ground traces, resulting from this power/ground voltage noise. Commonly, many output drivers are sourced by a given power/ground trace. The total noise effects are cumulative, in that, the total time current derivative (dI/dt) is often the sum total of the dI/dt of each of the several high current buffers which may transition simultaneously in the given application.
For some applications, such as wireless communications, the resulting EMI noise can even result in a loss of reception. For example, in the wireless communication context, a call can be lost solely due to EMI associated with a power/ground trace within the entire wireless circuitry. In addition, the propagation delay through all of the preceding circuitry and the final output driver inherently become a design consideration in attempting to deal with the undesirable dI/dt problem. The propagation delay is one design constraint that inherently competes with conventional methods that seek to minimize the effect of a high dI/dt through the final output driver.
Present solutions fail to provide a circuit that delivers charge to the gate of the output driver transistor in such a manner so as to cross the driver transistor""s turn-on threshold in the desired controlled manner, while at the same time avoiding the penalty of increased propagation delay. Currently there is not a circuit that provides for a very quick approach to the turn-on threshold within metal oxide semiconductor field effect transistor (MOSFET) technology that is commonly employed in many applications requiring low noise.
The rate of change in output drive current (dI/dt) is determined by the drive strength of the pre-driver transistor that is turned on during the turn-on phase of the associated output driver transistor. To adequately limit the maximum dI/dt, which occurs under best case conditions, a sufficiently slowed down pre-driver, using conventional methods, may produce a significant delay as compared to a pre-driver not so constrained. Such a delay is a substantial reduction in performance for many high speed applications.
Several conventional solutions have been presented to address pre-driver transition problems and try to provide a pad pre-driver speed improvement. One conventional solution is for a given output driver, the dI/dt is controlled by designing the pre-driver such that its output voltage crosses the threshold of the output transistor gradually. Here, the pre-driver is typically sized such that it charges the gate of the output driver at a predetermined and desired rate. However, as mentioned above, the propagation delay of the entire circuitry is significantly compromised. Another conventional solution connects a current source into a diode-connected device in series with a capacitor to generate a controlled charge rate for the gate of the output driver, offset by a threshold voltage. However, this conventional method suffers, in that, the required capacitor to perform this function is typically large when compared to the output driver gate capacitance. In addition, a diode-connected device is typically not strong enough to provide an offset lower than the threshold of the output transistor. The undesirable result is that the output transistor turns on abruptly. Any conventional solution that seeks to employ a slew rate control circuitry responding to the time voltage derivative (dV/dt) of the output pad cannot sufficiently address the associated dI/dt problems because dV/dt inherently lags dI/dt in such applications.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of ordinary skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.